Curve nodes have no line showing
I have seen this issue before, I believe it is a bug in the UI theme, since it only seems to happen with certain bundled themes

After fumbling around with the settings I could not find any way to fix it, or what color to change, so this is more of a workaround than an actual fix.

For the time being just change the current theme in the user preferences, either permanently or temporarily while working with nodes.